[PATCH v11 18/25] mm: Update can_follow_write_pte() for shadow stack

[Yu-cheng Yu]

Can_follow_write_pte() ensures a read-only page is COWed by checking the
FOLL_COW flag, and uses pte_dirty() to validate the flag is still valid.

Like a writable data page, a shadow stack page is writable, and becomes
read-only during copy-on-write, but it is always dirty.  Thus, in the
can_follow_write_pte() check, it belongs to the writable page case and
should be excluded from the read-only page pte_dirty() check.  Apply
the same changes to can_follow_write_pmd().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
v10:
- Reverse name changes to can_follow_write_*().

 mm/gup.c         | 8 +++++---
 mm/huge_memory.c | 8 +++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ae096ea7583f..f6a640f2ad57 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -384,9 +384,11 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
  * but only after we've gone through a COW cycle and they are dirty.
  */
-static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
+					struct vm_area_struct *vma)
 {
-	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
+	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte) &&
+				  !arch_shadow_stack_mapping(vma->vm_flags));
 }
 
 /*
@@ -439,7 +441,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	}
 	if ((flags & FOLL_NUMA) && pte_protnone(pte))
 		goto no_page;
-	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
+	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags, vma)) {
 		pte_unmap_unlock(ptep, ptl);
 		return NULL;
 	}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a580b5fb6e1a..0f4ed61a77f1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1296,9 +1296,11 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
  * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
  * but only after we've gone through a COW cycle and they are dirty.
  */
-static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags,
+					struct vm_area_struct *vma)
 {
-	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
+	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd) &&
+				  !arch_shadow_stack_mapping(vma->vm_flags));
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1311,7 +1313,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
-	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
+	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags, vma))
 		goto out;
 
 	/* Avoid dumping huge zero page */
-- 
2.21.0